Hydraulic control system

ABSTRACT

A hydraulic control system for an elevator or other similar reciprocating user is disclosed, which comprises an adjustable throttle valve and a control or bypass valve disposed in the fluid supply line between the pump and elevator. A pressure difference balance is provided for controlling the operation of the control valve, and the pressure difference balance has one end operatively connected to the pressure provided by the pump, and the other end operatively connected to the pressure in the user or load line. The control valve is thus adapted to maintain a desired pressure difference across the throttle valve which is independent of the load on the user. In addition, the zero setting of the pressure difference balance is adjustable, so as to permit the system to be equally operable in both the upward and downward movements of the elevator.

The present invention relates to a hydraulic control system adapted tocontrol hydraulically operated elevators and other users.

Hydraulic control systems are known which include a metering or throttlevalve positioned in the fluid supply line leading to a user, and apressure difference balance adapted to monitor the difference inpressure on opposite sides of the throttle valve and to generate ahydraulic control pressure as a function of the pressure difference. Thepressure difference balance comprises a piston which is biased at oneend by the pressure upstream of the throttle valve, and which is biasedat the other end by the pressure downstream of the valve. The systemalso includes a flow control valve which is positioned in the hydraulicsupply line between the pump and the throttle valve, and which iscontrolled by the control pressure generated by the pressure differencebalance to control the fluid pressure in the supply line upstream of thethrottle valve. A system of this general type is further described inGerman OS No. 21 39 119.

In the above control system, the flow control valve includes a controlchamber which receives the fluid from the supply line via a throttle,and which is connected via the control edge of the pressure differencebalance to a storage tank. As a result, there is a continuous flow offluid through the control chamber and to the storage tank, even in thestatic condition of the system. Thus the system is subject to fluidloss.

The above system is unsuitable for hydraulic control systems which areconnected to an intermittently operating pump. This applies primarily tohydraulically operated elevators, in which the pump operates only duringupward movement and the pump is inoperative during downward movement.The hydraulic control system serves the purpose of imparting a definedspeed to the elevator descending under its own weight, through acorresponding control of the fluid (i.e. oil) flowing out of theelevator cylinder. Also, very low "creeping speeds" are desirable uponthe approach of the predetermined end positions. However, this is notpossible since the fluid is lost from the control chamber, causing theadjusted creeping speed to increase, which is undesirable.

In accordance with the present invention, the above disadvantages areavoided by providing the pressure difference balance with a double edgecontrol which permits the control chamber of a control valve to beconnected either to a reference input line, or to a return flow line. Inthe disclosed embodiment, the fluid flows only during a hydraulicadjustment of the control valve, and this flow is insignificant withregard to its power loss and its fluid consumption. In the staticoperation of the control valve, no fluid flows into or out of thecontrol chamber.

When using a hydraulic control system in accordance with the presentinvention, and in particular in association with hydraulically operatedelevators, the control valve is preferably designed as a bypass valve.To control the pressure in the supply line at the adjustable throttle,the control valve connects the supply line with a return flow lineleading to a storage tank. The valve includes a regulating piston whichis biased on one side by the supply pressure and a spring, and on itsother side by the control pressure from the pressure difference balance.When used for this purpose, an advantage of the control valve resides inthe fact that it is suitable for both the upward and downwardoperations, without any further adjustment.

As one aspect of the present invention, the pressure difference balanceis connected to a reference input line. The pressure in this referenceinput line is converted to a control pressure as a function of thepressure difference which is present on the balance piston. Thereference input line may be connected with the supply line between thepump and the adjustable throttle valve, or to the user or load line,i.e. between the throttle valve and the user. In one embodiment, whichis adapted for use in association with reciprocatory users, inparticular elevators, the reference input line of the pressuredifference balance is connected via a two-way valve with either the userline downstream of the throttle valve, or with the supply line upstreamof the throttle valve. As a result, it is provided that the higherpressure is always available for the control of the control valve. Thisfeature is of particular importance when the elevator is at a standstillor during a slow descent, and the supply line between the control valveand the adjustable throttle valve is substantially at zero pressure. Inthis event, such low pressure would not be adequate to operate thecontrol valve.

When used with reciprocating users, and in particular elevators, thepresent invention further provides that the balance piston of thepressure difference balance may be mounted with an adjustable springtension and so that the zero position of the balance piston may beadjusted. Zero position is here defined as the position the balancepiston assumes when neither of its ends is biased by fluid pressure. Thespring tension may be adjusted so that the balance piston connects theoutput control line leading to the control valve with the referenceinput line in one zero position, and overlaps or slightly connects theoutput control line with the return flow line leading to the tank in theother zero position. Thus, during upward operation, the control valvemay be biased by the reference pressure so as to close the bypass andbuild up a pressure in the supply line, until the pressure has overcomethe initial force of the spring and the load pressure acting on thebalance piston. The balance piston then moves to close the outputcontrol line to the reference input line, or if necessary, to open thecontrol line to the return flow line. In the downward operation, theload pressure in the user line is initially greater than the pressure inthe supply line. As a result, the load pressure displaces the balancepiston against the initial force of the opposite spring, and the outputcontrol line is thereby connected with the reference input line, whichtends to close the control valve and increase the pressure in the supplyline. As the pressure increases in the supply line, it becomes operativeto act against the load pressure so that the balance piston closes thecontrol line to the reference input line and possibly opens the controlline to the tank, so that the control valve is again actuated in thesense of bypassing fluid to the tank and lowering the pressure in thesupply line.

A stop may be provided for limiting the movement of the balance piston,and in particular, for allowing a throttled opening between thereference input line and the control line. As a result, a dampening ofthe movement of the control valve may be provided.

The present invention also provides for the adjustability of the settingof the springs of the balance piston, which permits the hydrauliccontrol system to be operative with the same structural units in boththe upward and the downward directions. The setting of the springs ispreferably adjusted as a function of the elevator control, and in apreferred embodiment, the end abutment for one of the springs is mountedfor movement between two positions. Preferably, the movable end abutmentis in the form of a hydraulically operated piston, which ishydraulically adjustable by the elevator control as a function of theoperating direction.

A special advantage of the present invention resides in the fact thatthe limits of the adjustable movement of the abutment piston may beadjusted in each direction by adjustable mechanical stops. Thisarrangement permits a very accurate adjustment of the two zero positionsof the piston and of the operative spring forces. Thus the pressureratio of the throttle valve which is regulated by the pressuredifference balance may be adjusted independently of each other for boththe upward movement and the downward movement of the elevator.

When used in an elevator control, the throttle valve may also behydraulically controlled, in that it may be biased in one direction bythe supply pressure upstream of the valve, and in the other direction bya controllable counterpressure. To enable a low counterpressure, and toensure an automatic adjustment of the throttle valve even at a highsupply pressure, the throttle valve may be designed to include adifferential metering piston, with the small piston end being biased bythe supply pressure and the large piston end being biased by thecontrollable counterpressure.

In a preferred embodiment, the counterpressure for the metering pistonof the throttle valve is provided by the load pressure, via thepreviously mentioned two-way valve. In particular, the small end of themetering piston is provided with an annular groove immediately upstreamof its seat, and the groove is connected to the two-way valve via apilot duct in the piston and a second annular groove on the piston. Thisdesign of the metering piston provides that the load pressure is appliedto the counterpressure side of the piston in the direction of closing,and to the balance piston of the pressure difference balance in thesense of closing the control valve, before the metering piston hasopened to connect the supply line and the user line. Thus a pressurecorresponding to the pressure in the user line may build up beforecommunication is effected between the supply line and the user line.

The connection between the counterpressure chamber of the throttle valveand the two-way valve includes an inlet throttle. Further, thecounterpressure chamber is connected via a discharge throttle and stopvalve with the storage tank. By opening the stop valve, and by reason ofthe predetermined ratio between the inlet throttle and the dischargethrottle, the metering piston may be hydraulically actuated.

A further aspect of the hydraulic control system of the presentinvention resides in the fact that the pressure difference balance maybe adjusted to a certain pressure difference between the user line andthe supply line. As a result, the behavior of the user in operationsubstantially depends on the movement of the metering piston. Thismovement is predetermined by the inlet throttle and the dischargethrottle, so that a load independent behavior in operation, togetherwith uniform acceleration and deceleration, may be achieved.

Some of the objects and advantages of the present invention having beenstated, others will appear as the description proceeds, when taken inconjunction with the accompanying drawings, in which--

FIG. 1 is a schematic diagram of a hydraulic control system whichembodies the features of the present invention; and

FIG. 2 is a more detailed view of a portion of the circuit shown in FIG.1.

Referring more specifically to the drawings, FIG. 1 illustrates ahydraulic control system in accordance with the present invention andwhich is adapted to operate a user 1, such as an elevator, whichcomprises a cylinder 3 and a piston 2. A hydraulic pump 4 is powered bya motor 5, and the hydraulic fluid (oil) is delivered from the storagetank 6 and pumped into the supply line 7. From the supply line 7, thefluid passes through an adjustable throttle valve 8, and then to a userline 7A which is operatively connected to the user 1. The throttle valve8 thus is positioned to adjust the flow from the pump 4 to the user 1. Acontrol valve 9 is disposed in the supply line 7 between the pump 4 andthe throttle valve 8, for selectively bypassing a portion of the fluidin the supply line 7 to a return flow line 13 leading to the tank 6.

The control valve 9 includes a cylindrical housing having an inletopening communicating with the supply line 7 and an outlet openingcommunicating with the return flow line 13. A regulating piston 10 isslideably mounted in the housing for movement between a closed positionclosing communication between the inlet opening and outlet opening, andan open position which permits communication therebetween. The forwardportion of the piston includes an extension 11 of reduced diameter,which has a transverse groove which provides the connection between thesupply line 7 and return flow line 13. The forward extension 11 isbiased toward the open position by a spring 12 as well as the pressurein the supply line 7. The larger portion of the piston 10 is biased bythe pressure in a control chamber 14 formed at the inner end of thevalve housing, and if desired, an automatic pressure relief device (notshown) may be provided in the control chamber 14. Such a pressure reliefdevice is particularly desirable when the motor 5 has a Y-deltaelectrical circuit, and which starts on the Y circuit.

The supply line 7 further includes a one-way valve 16, which closes whenthe pump 4 is idle, i.e. when the elevator is idle or moves in adownward direction.

The pressure in the control chamber 14 is controlled via an outputcontrol line 15 leading from a pressure difference balance 17. Thebalance 17 includes a balance piston 18, which is slideably mounted in atubular cylinder between springs 22 and 23. Also, the cylindercommunicates with a reference input line 31, the return flow line 13,and the output control line 15. In operation, the balance 17 acts toselectively connect the reference input line or the return flow line tothe output control line 15, in response to the pressure difference onopposite sides of the throttle valve 8. The abutment for the spring 23is in the form of a forward end portion 24 of an abutment piston 25, andpressure may be applied to the opposite side of the piston 25 in theadjusting pressure chamber 28, via the adjustment line 27 and valve 30.The pressure in the chamber 28 is relieved via a throttle passage 26which extends through the piston. The chamber formed on the forward sideof the piston 25 is connected via the return flow line 13 to the tank 6.One end position of the piston 25 is determined by the adjustable setscrew 29, and the other end position is determined by the set screw 56.

The ends of the balance piston 18 define hydraulic control chambers, 19and 20, with the end chamber 19 receiving the pressure in the supplyline 7 via the pilot duct 33, and with the load pressure end chamber 20being biased, via the user pressure line 34, by the load or consumerpressure. Thus the balance piston performs a control motion as afunction of the ratio of the load pressure and supply line pressure,wherein the control shoulder 35 of the piston cooperates with the outletto the control line 15, the outlet 21 to the reference input line 31,and the outlet to the return flow line 13. The reference input line 31is connected to a two-way valve 32, which in turn is connected to thepilot line 33 of the supply line 7, and to the user pressure line 34.The respectively higher pressure of these two lines is supplied via thereference input line 31 to the pressure difference balance as the inputpressure thereto.

Referring now to the throttle valve 8, reference is made to FIG. 2. Thethrottle valve 8 has an inlet communicating with the supply line 7 anddefining an annular seat 40, and an outlet communicating with the userline 7A. A metering piston 36 is slideably mounted in the housing, andthe piston 36 includes a smaller forward cylindrical extension 37 havinga transverse control groove 38 through which a connection is madebetween the supply line 7 and the user line 7A. The forward extension 37further includes an annular shoulder 39 which is adapted to seat againstthe valve seat 40. This seating engagement permits the piston 36 toclose the user line 7A to the supply line 7 without substantial leakage,which is particularly important in the idle condition, so as to preventan unintended descent of the user, i.e. the cage of the elevator. Theforward extension 37 of the piston further includes an annular groove 41which is adjacent the shoulder 39, and which is connected via the pilotduct 42 in the piston with an annular groove 43 at the larger end 44 ofthe piston 36. The annular groove 43 is enclosed by suitable dynamicseals, and is connected with the user pressure line 34 which leads onthe one hand to the two-way valve 32, and on the other hand to thepressure difference balance 17 and adjusting valve 30. The particularplacement of the annular groove 41 on the piston 36 provides that theload pressure in the user line 7A is delivered via the duct system 42,34 to the pressure difference balance 17 immediately after the meteringpiston has lifted from the seat 40.

The rear end of the metering piston 36 includes a relatively largeshoulder 44, which is biased by a spring 45, and the metering pistonfurther includes a connecting duct 46 which passes through the pistonand communicates with the chamber 48 at the inner end of the valvehousing. The connecting duct 46 includes a throttle 47, so as topressurize the chamber 48 of the valve 8 with the user line pressure,when the valve is seated against the seat 40. This provides that thepiston is held against its seat, free from leakage, during idle orwhenever the pump is shut down.

The hydraulic control of the metering piston 36 includes a pressureconverter generally indicated at 52, and as best seen in FIG. 2. Thepressure converter 52 includes an inlet throttle 49, a dischargethrottle 50, a one-way valve 55, and a seat valve 51, which permitspassage to the return flow line 13 via the discharge throttle 50. Thecontrol line 53 from the throttle valve is connected via the inletthrottle 49 to the reference input line 31, and via seat valve 51 anddischarge throttle 50 with the tank 6. The inlet throttle 49 and thedischarge throttle 50 may be adjusted to a constant oil flow, and theyare therefore preferably constructed as adjustable flow controlregulators. Upon the adjustment of the flow ratio, the control pressurein the chamber 48 is dependent on the pressure in the reference inputline 31, and in this regard, it will be understood that the throttle 47in the metering piston 36 is very small in comparison to the throttle49.

The operation of the illustrated hydraulic control system will now bedescribed. During idle conditions, the motor 5 and the pump 4 areinoperative, the user 1, i.e. the elevator cage, exerts a pressure inthe user line 7A, which is applied via the duct 46 and throttle 47, tothe control chamber 48 adjacent the large end of the metering piston 36.As a result, the shoulder 39 is held against the seat 40, and the userline 7A is closed to the supply line 7, substantially free of leakage.Also, the supply line 7 is substantially at zero pressure, as is theannular duct 41 in the metering piston 36. The one-way valve 55 in thepressure converter 52 prevents the oil from returning from the controlchamber 48 via line 53 into the control system. For this reason, thecontrol chamber 14 of the control valve 9 is also at zero pressure. Thepiston 10 thus serves to open the supply line 7 to the return flow line13, by reason of the force of the spring 12.

To initiate upward movement, the motor 5 and pump 4 are put intooperation, and the valve 51 is switched. The valve 30 remains in theindicated position. In this regard, further non-illustarted connectingpossibilities for the control of the startup acceleration and for thecontrol of the creeping speed upon approaching a stopped position, arepossible. Also not considered is the possibility of a pressure relief inthe control chamber 14 when the motor 5 is initially started with aY-connection.

Since the user pressure line 34 and the supply pressure pilot line 33are initially at zero pressure, the control chamber 14 also is under nopressure. The spring 12 pushes the piston 10 against the stop screw 54,which is adjusted so that the oil flow is throttled and a pressure ofabout 3-6 bar develops in the supply line 7. This supply pressure isapplied via pilot line 33 to the balance piston 18 on the supplypressure end 19. Similarly, the supply pressure reaches the two-wayvalve 32 and the reference input line 31, and the pressure converter 52.Also, the supply pressure passes through the converter 52 to the line 53and the control chamber 48 of the throttle valve 8. In this regard, theinlet throttle 49 and the discharge throttle 50 are adjusted so that theoil flow through the inlet throttle 49 is about half as much as the oilflow through discharge throttle 50. As a result, the metering piston 36is relieved of pressure in the chamber 48, and it is moved to the rightby the pressure in the supply line 7. As it does so, it displaces theoil from the chamber 48 via the discharge throttle 50.

Upon the metering piston 36 moving to the right and lifting from theseat 40, the annular duct 41 becomes connected with the user line 7A.The pressure of the load is therefore supplied, via the annular duct 41and the pilot duct 42, into the annular groove 43. From the groove 43,the pressure is supplied through the user pressure line 34 to thetwo-way valve 32, and to the load pressure end 20 of the pressuredifference balance 17.

Since the valve 30 remains closed, the abutment piston 25 rests againstthe set screw 29. The springs 22, 23 are so dimensioned that the tensionof the spring 22 preponderates in this position, and is operative tomove the balance piston 18 in the direction of the stop 57. Since theload pressure end 20 of the balance is simultaneously biased in the samedirection, the balance piston is held against the stop 57. As a result,the outlet to the reference input line 31 communicates with the outputcontrol line 15, and the control chamber 14 of the flow control valve 9is biased by the pressure in the reference line 31. This pressure isselected by the two-way valve 32 to be the higher of either the pressurein the supply line 7, or the pressure in the user line 7A.

The pressure in the chamber 14 causes the control piston 10 of the flowcontrol valve to move to the left, as seen in FIG. 1, so that the supplyline 7 is closed with respect to the return flow line 13. This permitsthe pressure in the line 7 to build up further. Since the pressure inthe line 7 is also supplied to the balance piston on the supply pressureend 19 via the pilot line 33, the pressure counteracts spring tension 22and the load pressure on the end 20, so that the output control line 15is initially closed to the reference line connection 21, and thenconnected with the return flow line 13. As soon as the pressure gradientbetween the load pressure end 20 and the supply pressure end 19, andthus also the pressure gradient between the user line 7A and the supplyline 7, becomes too large, i.e. greater than that provided by the springtension which is operative in a direction toward the stop 57, thebalance piston moves to the left and thus connects the control chamber14 of the control valve 9 with the return flow line 13. This provides alarger discharge cross section between the line 7 and the return flowline 13, until the pressure gradient has regulated itself to the desiredvalue. The pressure drop across the metering piston 36 remains constantduring the upward movement, whereby the flow is determined only by thecross section of the opening of the metering piston, and is independentof the load pressure. The entire behavior of the movement is thussubstantially determined by the motion of the metering piston 36. Sincethis motion is load independent, in that the inlet flow regulator 49 andthe discharge flow regulator 50 ensure a constant oil flow, the piston 2of the user 1 moves in a load independent manner with uniformacceleration and deceleration.

Upward movement of the user 1 is terminated by closing the valve 51. Aspreviously indicated, means and circuits may be provided for producing acreeping movement before reaching the terminal position, which are notillustrated. After the valve 51 has been closed, the motor 5 is alsodisconnected after a certain delay. Thus all elements are in the idleposition, as shown in the drawing. Specifically, the user line 7A isagain closed in a leakproof manner by the shoulder 39 resting upon theseat 40, and the pressure of the user line again builds up in thecontrol chamber 48. The one-way valve 55 prevents the oil from returningfrom the control chamber 48 into the precontrol portion of the circuit.

It should be noted that the pressure gradient between the supply line 7and the user line 7A may be predetermined by the setting of the screw 29of the abutment piston 25. This permits the flow of the throttle valveto vary by nearly a 1:2 ratio. As a result, one and the same embodimentof the throttle valve may be employed for a wide range of applications.

To initiate the lowering operation, the valves 30 and 51 areconcurrently switched to the open position. The motor 5 and the pump 4remain inoperative, and the one-way return valve 16 which connects thepump with the supply line 7 is closed in the direction toward the pumpby the associated spring.

The opening of the valve 51 relieves the control chamber 48 of pressure,and the metering piston 36 moves under the pressure of the user line 7A,which is operative on the opposite annular surface of the piston 36, tothe right as seen in FIG. 2. The piston 36 thus lifts from the seat 40,and the load pressure in the line 7A is supplied via the annular duct41, pilot duct 42, annular groove 43 and user pressure line 34, to thetwo-way valve 32. The pressure moves through the valve 32 to thereference input line 31 and to the pressure converter 52. The line 31leads to the outlet 21 of the pressure difference balance 17, and thepressure is also delivered via the line 34 to the valve 30 and to theload pressure end 20 of the pressure difference balance.

From the valve 30, the pressure is supplied to the pressure chamber 28of the abutment piston 25, and the abutment piston is displaced to theleft as seen in FIG. 1. As a result, the zero position of the balancepiston 18 is displaced to the left until the abutment piston 25 engagesthe stop 56. The stop 56 is an adjustable set screw, so that the controlshoulder 35 of the balance piston covers the output control line 15 inits zero position, or already opens the line 15 to the return flow line13.

It should be particularly noted that the adjustment of the set screw 56for the piston 25 provides for the pressure ratio across the meteringvalve to be adjusted during the downward movement, independently of thatof the upward movement. As previously noted, the pressure ratio for theupward movement may be adjusted by the screw 29.

The supply pressure on the supply pressure end 19, as well as thetension of the spring 23, which is biased by the displacement of thepiston 25, tend to move the balance piston to the left, as seen inFIG. 1. Acting in the opposite direction is the load pressure in theload pressure end 20, and the spring 22. During the lowering operation,the load pressure is higher than the supply pressure, and as a result,the balance piston is displaced to the right, as long as the meteringpiston 36 is closed. Thus the pressure in the reference line 31 reachesthe output control line 15 and the control chamber 14 of the controlvalve, via the pressure difference balance. This acts to close thebypass from the supply line 7 to the return flow line 13, against theforce of the spring 12. When the throttle valve 8 opens, a pressurebuilds in the supply line 7, and this increase in pressure causes thebalance piston 18 to move to the left, so that the output control line15 is first closed, and then opened to the return flow line 13 as thepressure builds further. Thus, the pressure in the control chamber 14decreases. The pressure difference balance 17 thus regulates a constantdrop in pressure across the throttle valve 8, and as a result thedownward movement is only dependent on the cross section of the openingat the metering piston 36. This opening cross section is againdetermined by the adjustment of the inlet flow regulator 49 and thedischarge flow regulator 50, and a load independent downward movementmay thus be ensured.

In the drawings and specification, there has been set forth a preferredembodiment of the invention, and although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation.

That which is claimed is:
 1. A hydraulic control system for delivering apressurized hydraulic fluid to a user (1) and comprising:pump means(4,5) for supplying pressurized hydraulic fluid to a fluid supply line(7), a user line (7A) operatively connected to said user (1), adjustablethrottle valve means (8) connected between said fluid supply line andsaid user line for controlling the flow of the hydraulic fluid from saidpump means to said user, pressure difference balance means (17)including a reference input line (31), a return flow line (13), and anoutput control line (15), for selectively connecting said referenceinput line or said return flow line to said output control line inresponse to the pressure difference in said fluid supply line and saiduser line, and control valve means (9) disposed in said fluid supplyline between said pump means and said throttle valve means andoperatively connected to said output control line of said pressuredifference balance means for selectively exhausting a portion of thefluid in said fluid supply line in response to the fluid pressure insaid output control line.
 2. The hydraulic control system as defined inclaim 1 wherein said control valve means comprises a housing having aninlet opening communicating with said fluid supply line and an outletopening communicating with said return flow line (13), a piston (10)slideably mounted in said housing for movement between a closed positionclosing communication between said inlet opening and said outletopening, and an open position permitting communication therebetween, anda control chamber (14) in said housing on one side of said piston andoperatively connected to said output control line (15) of said pressuredifference balance means such that the pressure in said output controlline acts to bias said piston into a position between said closedposition and said open position.
 3. The hydraulic control system asdefined in claim 2 wherein said control valve means further comprisesspring biasing means (12) mounted on the other side of said piston andsuch that said spring biasing means and the pressure of the fluid insaid fluid supply line act to bias said piston toward said openposition, and the pressure in said control chamber (14) acts to biassaid piston toward said closed position.
 4. The hydraulic control systemas defined in claim 3 wherein said pressure difference balance meanscomprises a cylinder slideably mounting a balance piston (18) therein,and with said reference input line (31), said return flow line (13), andsaid output control line (15) each communicating with said cylinder. 5.The hydraulic control system as defined in claim 4 wherein said pressuredifferential balance means further comprises a line (33) connecting oneend (19) of said cylinder to said fluid supply line for biasing saidbalance piston to a position so as to interconnect said output controlline and said return flow line, and a user pressure line (34) connectedto the opposite end (20) of said cylinder and adapted for biasing thebalance piston to a position so as to interconnect said reference inputline (31) and said output control line.
 6. The hydraulic control systemas defined in claim 5 wherein said control system further comprisestwo-way valve means (32) for connecting said reference input line (31)to either said fluid supply line (7) or said user pressure line (34) inaccordance with which line is at the higher pressure.
 7. The hydrauliccontrol system as defined in claim 6 wherein said throttle valve meansincludes duct means (42) for selectively connecting said user line (7A)to said user pressure line (34).
 8. The hydraulic control system asdefined in claim 7 wherein said pressure difference balance meansfurther comprises means for selectively positioning said balance pistonat either a first zero position wherein the reference input line (31)communicates with said output control line (15), or at a second zeroposition wherein said output control line is either closed or incommunication with said return flow line (13).
 9. The hydraulic controlsystem as defined in claim 8 wherein said means for selectivelypositioning said balance piston includes springs (22, 23) supportingsaid piston at respective ends of said cylinder, and means forselectively altering the tension of one of said springs.
 10. Thehydraulic control system as defined in claim 9 wherein said pressuredifference balance means further includes a stop (57) for limitingmovement of said balance piston in a direction toward said first zeroposition.
 11. The hydraulic control system as defined in claim 10wherein said means for selectively positioning said balance pistoncomprises an abutment piston (25) coaxially mounted in said cylinderadjacent one end of said balance piston, with a spring (23) interposedtherebetween, and means (30) for selectively moving said abutment pistonin a direction toward said balance piston and so as to move said balancepiston toward said second zero position.
 12. The hydraulic controlsystem as defined in claim 11 wherein said means for selectivelypositioning said balance piston further comprises means (29,56) foraccurately adjusting the end positions of the movement of said abutmentpiston.
 13. The hydraulic control system as defined in claim 6 whereinsaid throttle valve means comprises a valve housing having an inletcommunicating with said fluid supply line and defining an annular seat(40), an outlet communicating with said user line, and a metering piston(36) slideably mounted in said valve housing so as to selectively moveaxially into engagement with said seat to close communication betweensaid inlet and said outlet.
 14. The hydraulic control system as definedin claim 13 wherein said metering piston (36) comprises a cylindricalfront extension (37) adapted to extend through said annular seat (40) inthe closed position, and said front extension includes an annular groove(41) therein which is located on the supply side of said seat in theclosed position, and wherein said throttle valve means further includesa pilot duct (42) extending through said metering piston to a rearannular groove (36) on said metering piston and then to said userpressure line (34).
 15. The hydraulic control system as defined in claim14 wherein said metering piston of said throttle valve means furtherincludes a rear cylindrical shoulder (44) which is larger than saidcylindrical front extension (37) to define a rear pressure chamber (48)between said rear shoulder and the end of said valve housing, andwherein a pilot duct (46) communicates between said user line and saidrear pressure chamber (48), with said pilot duct (46) including athrottle (47).
 16. The hydraulic control system as defined in claim 15wherein said pilot duct (46) extends through said metering piston. 17.The hydraulic control system as defined in claim 15 wherein saidthrottle valve means further includes a line (53) communicating betweensaid rear chamber (48) and said return flow line (13) via a controlvalve (51) and a flow regulator (50).
 18. The hydraulic control systemas defined in claim 17 wherein said line (53) further communicates withsaid reference input line (31) via a one-way valve (55) which precludesflow in a direction toward said line (31) and a second flow regulator(49) which permits a flow rate greater than that of said first mentionedflow regulator (50).
 19. A hydraulic control system for delivering apressurized hydraulic fluid to a user (1) and comprising:pump means(4,5) for supplying pressurized hydraulic fluid to a fluid supply line(7), a user line (7A) operatively connected to said user (1), adjustablethrottle valve means (8) connected between said fluid supply line andsaid user line for controlling the flow of the hydraulic fluid from saidpump means to said user, pressure difference balance means (17)including a reference input line (31), a return flow line (13), and anoutput control line (15), for selectively connecting said referenceinput line or said return flow line to said output control line inresponse to the pressure difference in said fluid supply line and saiduser line, and wherein said return flow line is connected to a storagetank (6), with said storage tank being operatively connected to saidpump means for supplying hydraulic fluid thereto, means (32, 42) forselectively connecting said reference input line to either said supplyline or said user line, and control valve means (9) disposed in saidfluid supply line between said pump means and said throttle valve meansand operatively connected to said output control line of said pressuredifference balance means for selectively exhausting a portion of thefluid in said fluid supply line in response to the fluid pressure insaid output control line.
 20. The hydraulic control system as defined inclaim 19 wherein said control valve means includes an outletcommunicating with said return flow line (13), such that the portion ofthe fluid which is exhausted from said fluid supply line passes intosaid return flow line passes into said return flow line and to saidstorage tank (6).
 21. The hydraulic control system as defined in claim20 wherein said pressure difference balance means comprises a cylinderslideably mounting a balance piston (18) therein, and means forselectively positioning said balance piston at either a first zeroposition wherein said reference input line (31) communicates with saidoutput control line (15) or at a second zero position wherein saidoutput control line is either closed or in communication with saidreturn flow line (13).